Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic device while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and semiconductor lasers. Depending on the frequency of operation and power requirements, these devices have been conventionally made on silicon, gallium arsenide (GaAs), indium phosphide (InP), and in recent years gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors. Gallium nitride material systems give rise to microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and thermal conductivity that is greater than GaAs, InP, or silicon, and thus suitable for use in high power applications. GaN is also used in manufacturing of blue and ultraviolet lasers and light-emitting diodes. In spite of the high-temperature performance, GaN electronic and optoelectronic devices are limited in performance due to relatively low thermal resistance of the substrates commonly used for growth of GaN. This deficiency is most pronounced in high-power microwave and millimeter-wave transistors and amplifiers where reduced cooling requirements and longer device life, both benefiting from lower junction temperature, are in critical demand. Similar need is exhibited in high-power blue and ultraviolet lasers where a several-micrometer-wide laser cavity stripe dissipates power into the chip though low thermal conductivity materials.
It is well known that diamond is the most thermally conductive substance known to man. For this reason, the semiconductor industry has been employing diamond heat-sinks and heat spreaders for improved thermal management since the commercialization of synthetic diamond by chemical-vapor deposition in the 1980s. The objective of optimal heat management is to bring the diamond heat-spreader or diamond layers into close proximity of the heat source in the electronic or optoelectronic devices. This means building devices on thin chips and mounted on diamond heat-spreaders, coating devices with diamond layers, or transferring device epilayers onto diamond.
GaN-on-diamond technology and resulting devices (described in U.S. Pat. No. 7,595,507) involve structures which feature atomically attached GaN epilayers to diamond substrates. This technology enables bringing together the best heat conductor (diamond) together with electronic and optoelectronic devices based on gallium-nitride (GaN) and GaN-related compounds. Due to GaN's inherent high critical electrical field and wide bandgap, GaN devices are preferred for high power electrical and optoelectronic applications, such as, high power RF transistors and amplifiers, power management devices (Schottky diodes and switching transistors), as well as, high power blue and ultraviolet lasers or light-emitting diodes.
GaN is presently grown on several different substrates: sapphire, silicon, silicon carbide, aluminum nitride, single-crystal diamond, and GaN substrates. With the exception of GaN substrates, all other materials have lattice constants that differ from that of GaN and AlGaN. Natural diamond is an excellent thermal conductor, but has not been available for these applications due to scarcity and price. Presently, synthetic diamond is being manufactured with various degrees of crystallinity. Polycrystalline diamond deposited by chemical-vapor deposition (CVD) is suitable for use in the semiconductor industry as its thermal conductivity is close to that of single crystal diamond, while it still remains isolating, has low dielectric losses, and can be made transparent. CVD diamond substrates for semiconductor industry can be formed as round wafers with standard diameters. Diamond wafers are manufactured by chemical vapor deposition by one of three main methods: plasma enhanced diamond CVD where the energy to dissociate the reactants comes from a microwave source, thermally assisted diamond CVD where the energy for dissociating gasses comes from a hot filament, and plasma torch where ions are accelerated using a high DC voltage. In these processes, synthetic diamond is grown on top of non-diamond substrates, such as, silicon, silicon nitride, silicon carbide and different metals.
The CVD diamond growth process is carried out in a vacuum chamber within which a substrate is provided on top of which diamond is to be grown. The substrate is exposed to the energy source needed to dissociate molecules of precursor gases needed to form diamond on the surface of the substrate. The precursor gases needed in the chemical vapor deposition of diamond are a source of carbon diluted in hydrogen (H2). Typical carbon-carrying gases are methane (CH4), ethane (C2H6), carbon monoxide (CO), and acetylene (C2H2), with methane (CH4) being the most commonly used. The gas combination needed for efficient diamond deposition contains a small (several percent) composition of the carbon-carrying gas in hydrogen, and the reaction can be further assisted with addition of oxygen or oxygen precursors such as CO or CO2. A most common parameter specifying the gas-flow recipe is given in terms of the molar ratio of carbon carrying gas flow and hydrogen gas flow. For example, in terms of percentage [CH4]/[H2] where [CH4] and [H2] are molar flow rates typically measured in standard cubic centimetres per minute (sccm). Typical substrate temperatures during the deposition process are between 550° C. and 1200° C., and deposition rates are usually measured in micrometers (μm) per hour.
Growth of synthetic diamond on non-diamond substrates includes a surface preparation phase and nucleation phase in which conditions are adjusted to enhance the growth of diamond crystals on the host (non-diamond) substrate. This is most commonly done by scratching or “seeding” the surface with diamond powder in a controlled and repeatable manner. During the growth phase, the grain size of synthetic diamond increases and as a result synthetic diamond films are inherently rough after deposition. The nucleation of diamond generally starts with very small diamond domains embedded in non-diamond matrix which has poor thermal conductivity in the near-substrate regions.
There is a need in the industry to improve the thermal performance of electronic and optoelectronic devices that are integrated with synthetic diamond. The near-substrate thermal boundary resistance inherent to synthetic diamond growth is the final limitation to harnessing the high thermal conductivity of synthetic diamond for electronic and optoelectronic applications. The objective of embodiments of the present invention is thus to provide manufacturing processes that produce a diamond heat spreader directly attached to the active layers of electronic/optoelectronic devices with low near-substrate thermal boundary resistance. This specification describes methods for minimizing and optimizing thermal boundary resistance between epilayers of a semiconductor device and a bulk diamond substrate and semiconductor device structures fabricated using such methods to meet this objective. Embodiments relate to methods which involve growing polycrystalline CVD diamond material on a substrate comprising one or more semiconductor layers and particularly to wide band gap compound semiconductors, more particularly III-V semiconductors such as GaN.